Passive UHF RFID (radio frequency identification) protocols require the tag to be powered by the reader's field and to use the field to backscatter information on the same frequency. The technical term for such a system, where both the transmit and receive sections of the device are simultaneously operating on the same frequency is “homodyne.” One class of homodyne systems intends to only transmit a pure continuous sinusoidal wave (CW) signal while in the receive mode. UHF RFID reader systems are of this class. A challenge is presented to the homodyne systems when the receiver section is not well isolated from the transmitter section. Transmitter (TX) leakage into the receive (RX) path can be as much as 110 dB above the desired backscattered receive signal. Such a high TX leakage to receive signal ratio leaves the receiver section quite susceptible to typical nonlinearities associated with standard cost effective analog signal processing components. Therefore an unusually high dynamic range in the receiver section would be required.
Passive and semi-active (battery assisted) UHF RFID communications use radar cross section (RCS) modulation to send data from the transponder to the reader. That means the reader transmits a sinusoidal RF signal toward the transponder. Some of the RF energy which hits the transponder reflects back to the reader. By modulating its RCS, the transponder is able to communicate data back to the reader.
This presents many design challenges. In particular, the reader electronics must be designed to receive a very weak signal while it is transmitting a very strong signal at the same frequency. Whereas many other wireless communications schemes use frequency division multiplexing, the RFID reader cannot since its own transmit field is being used as a medium for communications from transponder to reader. The transmit signal may be 1 watt or more, while the receive signal for semi-active transponders (those which only use the RF signal for communications, not for power) may be as low as 1 picowatt (10−12 watt), e.g., 12 orders of magnitude less power. For passive transponders the receive strength is usually at least 1 nanowatt (1031 9 watt), which is still pretty challenging.